As memory densities increase, memories utilizing ceramic dielectrics become increasingly attractive because of the high dielectric constants provided by ceramics such as lead zirconate titanate (PZT). For example, in DRAMs, the data is stored as a charge on a capacitor. The length of time between refresh cycles is determined by the size of the capacitor. As the size of the memory cell is reduced, the size of the capacitor must also be reduced, thereby leading to a decrease in the time interval between refresh cycles. To maintain reasonable intervals between refresh cycles, the capacitance of the capacitors must be increased without increasing the physical size of the capacitors. The simplest method for obtaining higher capacitance is to use a dielectric medium with an increased dielectric constant.
In prior art systems, the memory capacitors and transistors are fabricated separately, both in time and space. The capacitors and transistors are then connected together during the standard CMOS metalization steps. While this method works adequately for memories up to 64 Mbytes, resulting cell size is too large to allow a cost effective memory to be made at higher densities.
To obtain higher densities, the ferroelectric capacitor must be built with the bottom electrode of the capacitor connected directly to the CMOS transistor. This geometry saves a significant amount of space, since the source of the transistor and the capacitor can be built in the same space. This fabrication technique requires the capacitor electrode to be in contact with the silicon structures of the source or the drain during the subsequent deposition of the ceramic dielectric and the sintering operations following that deposition. The temperatures involved in these steps together with the need to have oxygen present lead to the oxidation of the silicon structures. These oxidation reactions lead to high resistance regions that destroy the contact efficiency.
Broadly, it is the object of the present invention to provide an improved contact system for use in making connections to silicon structures that can withstand high temperatures in the presence of oxygen.
It is a further object of the present invention to provide an improved contact system for use in connecting ceramic capacitors directly to the gate or drain of a CMOS transistor.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.